Not too long ago, I wrote a column about all the work we do here at the Wildlife Conservation Society’s Queens Zoo to encourage our rare and endangered animal species to breed.

I talked about how we participate in cooperative breeding programs with other zoos throughout the country and how we analyze the genetics of each animal that participates in the breeding program to be sure we maintain healthy gene pools in breeding populations. I even wrote about some of the techniques we use to get pairs of animals interested in each other in order to produce youngsters.

But it occurs to me, that when I wrote that column, I only told half the story.

For every hour I spend trying to set up breeding pairs of animals, I probably spend another hour trying to keep certain other animals from doing the same thing. The reasons for this are pretty simple. We simply do not have enough space to have every animal in the zoo have babies every year.

If every animal that could reproduce did so, the zoo would very quickly be overrun with relatives of the best breeders, usually of the most common species. This would leave little room for any of the rare species or the offspring of individuals who have not yet contributed to the future by having babies. That’s why we work to reserve our space for rare or endangered species

Often times the job of keeping animals from reproducing is difficult because of certain limitations. Seemingly, the easiest thing to do would be to just separate animals from each other, thus reducing their opportunities to breed. Many animals spend most of their adult lives alone and do quite well living a solitary lifestyle.

Unfortunately, people do not see it that way, and even if an animal would quite happily live its life alone in the wild, when people see them alone at the zoo they feel the animals look lonely. No matter how I try to explain to these very sympathetic zoo visitors that some animals like to live alone, exhibiting individual animals alone makes some zoo visitors feel bad.

And zoo visitors feeling bad makes the zoo director feel bad, so we try to avoid exhibiting solitary animals whenever we can. We may try to keep single sex pairs or groups (that is, all males or all females), so the animals have companions - like we do with our all male sea lion group - but many species can’t be kept together with multiple males or multiple females without fighting. Sometimes brothers or sisters can be kept together and some animals will not tolerate other adults living alongside their siblings – even after they grow up.

Some animals can be kept from breeding just by not providing them with nests or nesting material. For example, if cavity nesting birds, such as our scarlet macaws or wood ducks, are not provided with an adequate nest box to lay their eggs, they will not bother.

Finally, with some species we resort to contraception either through medication or procedures. By spaying and neutering we are able to keep males and females together without them having youngsters. This way, we can keep animals like our pronghorn females with their father long after they grow up, without risking reproduction.

The combined space of all the zoos and aquaria in the country would still not have nearly enough room to save most of the species of wildlife in danger of extinction. We need to be very selective about what animals we do breed in zoos and how they divide their space. So even though the business of breeding endangered species is not simple – it is almost always interesting.

Scott Silver is the director of the Wildlife Conservation Society’s Queens Zoo.